Fix 2bb5c9ac84: typo in disable script parameter randomisation helptext

Motivation / Problem

A typo introduced in 2bb5c9ac848d5ab3ebbe08ef82880ef406aad205.

Fixed the typo.

None

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

The bug fix is important enough to be backported? (label: 'backport requested')
This PR touches english.txt or translations? Check the guidelines
This PR affects the save game format? (label 'savegame upgrade')
This PR affects the GS/AI API? (label 'needs review: Script API')
- ai_changelog.hpp, game_changelog.hpp need updating.
- The compatibility wrappers (compat_*.nut) need updating.
This PR affects the NewGRF API? (label 'needs review: NewGRF')
- newgrf_debug_data.h may need updating.
- PR must be added to API tracker